An opening-degree adjustment valve which variably controls an opening area of a flow passage and which is provided in a flow passage between a suction port and a suction hole communicating with a cylinder bore, of a compressor of an air conditioning system for vehicles, etc., such as shown in FIG. 6, is known. In FIG. 6, symbol 100 implies an opening-degree adjustment valve, and opening-degree adjustment valve 100 having casing 101 and valve body 102 is provided in a flow passage, which is also called a suction passage, between a suction port of a compressor and a suction hole of a cylinder bore, where casing 101 is disposed in the suction passage and valve body 102 is contained in casing 101 movably. Casing 101 is formed in a shape of a cylinder having the bottom, and small opening section 103 and large opening section 104 are formed on a side surface of casing 101. Further, by spring 105 which is provided in casing 101, valve body 102 is urged in a direction of shutting opening 104, which means a direction of decreasing an opening area of the flow passage. Furthermore, even in a condition where valve body 102 is urged by spring 105 in the direction of decreasing the opening area of the flow passage, small opening section 103 is always opened without closing.
In such opening-degree adjustment valve 100, because gas in a suction chamber is sucked into the cylinder bore at the time of the compressor startup, etc., the pressure in the suction chamber decreases, and valve body 102 is pushed downward in FIG. 6, or in a direction of increasing the opening area of large opening section 104, and the opening area of the flow passage increases. On the other hand, when a fluid sucked from a suction port side is decreased at the time of the compressor shutdown, etc., valve body 102 is pushed upward in FIG. 6, or in a direction of decreasing the opening area of large opening section 104, by the bias force of spring 105. In addition, space 106 of which volume is varied by casing 101 and valve body 102 as valve body 102 moves is provided in opening-degree adjustment valve 100, and space 106 communicates with flow passages other than the space through communication path 107. When valve body 102 moves, because a gap is formed between valve body 102 and inner peripheral surface 101a of casing 101 so that refrigerant is flowed from communication path 107 into space 106 and flowed out therefrom, the resonance movement of valve body 102 comes to be restrained.
However, opening-degree adjustment valve 100 keeps an opening-degree which makes a differential pressure through opening-degree adjustment valve 100 be nearly constant. Therefore, when there exists liquid refrigerant in an evaporator of the air conditioning system for vehicle at the compressor startup, etc., it may be caused that opening-degree adjustment valve 100 operates as the compressor starts up so as to rapidly increase the opening area of the flow passage and the liquid refrigerant in the evaporator flows into the compressor at once, causing the liquid compression, so as to adversely affect the compressor. Further, a torque limiter might operate when a compressor is a clutchless compressor.
Furthermore, when the rotation speed of an engine as a drive source of the compressor increases as a vehicle runs, the discharge displacement of the variable displacement compressor increases temporarily. Until the discharge displacement has been controlled to decrease, especially in a case where the movement to control the displacement delays, the refrigerant flow rate is increased by the amount multiplied by the engine rotation speed increase and the discharge displacement, so that the power consumption increases. Therefore, there might be a bad influence on an engine control and an acceleration performance of the vehicle. As a reason why the movement to control the displacement of the compressor delays, it can be given that even if the refrigerant flow rate of the compressor increases when the engine rotation speed increases, because of the great volume of the refrigerant in the evaporator, the inner pressure fluctuation in the evaporator can be restrained and the suction pressure decrease also becomes slow. Furthermore, it can be also given as a reason why the movement to control the displacement of the compressor delays, that during a middle-low load operation, the discharge pressure decreases and the refrigerant flow rate from a discharge chamber for controlling the displacement decreases, so that the inner pressure increase in a crank chamber slows.    Patent document 1: JP-2001-289177-A